sedimentary
Chert (often called Flint when dark or found in chalk)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale. Color: Highly variable, typically grey, tan, or brown with white weathering. Luster: Dull to waxy. Crystal structure: Trigonal (microscopic). Cleavage: None (exhibits conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale. Color: Highly variable, typically grey, tan, or brown with white weathering. Luster: Dull to waxy. Crystal structure: Trigonal (microscopic). Cleavage: None (exhibits conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from seawater or the replacement of carbonate minerals in limestone. It frequently occurs as nodules or layers within sedimentary beds. Formation can range from the Precambrian to recent geological eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in road construction, as an abrasive, and occasionally as a laboratory mortar and pestle material.
Geological facts
Because it breaks with a conchoidal fracture, it produces edges that are sharper than steel. It was one of the most vital resources for prehistoric humans. It is chemically very resistant to weathering, which is why it is often found as loose stones in stream beds.
Field identification & locations
Identify in the field by its hardness (it will scratch glass and steel), its smooth curved fracture surfaces, and its resonance when struck. Found globally in limestone and dolomite formations, as well as in glacial till.
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